This invention relates generally to radio frequency circuits and more particularly to radio frequency distributed circuits.
As is known in the art, radio frequency distributed circuits are well known to provide various circuit functions particularly over broad bands of microwave frequencies. For example, such distributed circuits can be used to provide mixers, amplifiers, and switches. In particular, one type of distributed circuit commonly referred to as a distributed amplifier includes a plurality of successively interconnected active elements and has been shown to provide amplification of radio frequency signals.
Many examples of such distributed amplifiers are known. Modern distributed amplifiers have active elements provided as field effect transistors and particularly so-called metal semiconductor field effect transistors (MESFETs) fabricated on a Group III-V material, such as gallium arsenide. Earlier attempts in making distributed amplifiers prior to the advent of the field effect transistor used vacuum tubes. Both field effect transistors and, in particular, vacuum tubes lent themselves to use in distributed circuit applications since their input and output impedances are relatively high and primarily capacitive over a broad range of operating frequencies.
As it is known, distributed amplifiers are operative over broad bandwidths, and accomplish this by providing a network which includes the input and output impedances of each one of the active elements in combination with the input and output lines. By incorporating the input and output impedances into the propagation networks a suitable impedance characteristic is maintained over a broad range of operating frequencies.
In the past there have been attempts to make distributed amplifiers using bipolar transistors. These earlier attempts, in general, had not been very successful since the characteristics of a bipolar transistor do not lend themselves easily to incorporation into a distributed circuit topology. Generally bipolar transistors have relatively low input impedances which are primarily resistive rather than capacitive. Accordingly, it is not possible to obtain the equivalent distributed transmission line structure by incorporating the input impedance of the device with a transmission line.
One approach to providing such a bipolar distributed amplifier includes passive impedance matching networks disposed at the input and output of each one of the bipolar transistors. There are several problems with this approach. One problem is that the passive impedance matching networks have the undesired effect of reducing the bandwidth of the device. Since bandwidth is the most attractive attribute of the distributed amplifier, this drawback is quite undesirable. Furthermore, the use of passive impedance matching networks for each device would only serve to increase the size of the device as well as circuit complexity, thus making the amplifier more expensive and less reliable. Given the above limitations, it is apparent why bipolar implementations of distributed circuits and particularly distributed amplifiers were not widely used.
With modern monolithic microwave integrated circuits and operation at microwave and millimeter wave frequencies, the preferred approach for providing a distributed amplifier is to provide the distributed amplifier as a monolithic microwave integrated circuit. With MESFET field effect transistors as the active devices in such a circuit, one problem is that output power from the circuit is relatively low. This limitation in output power is the result of relatively low breakdown voltage characteristics of MESFETs, limited current capabilities, as well as the load impedance characteristics presented to MESFETs being strongly dependent upon the frequency of operation of the MESFETs and signal levels fed to the MESFETs. One approach known in the art for overcoming these problems is described in U.S. Pat. No. 4,543,535 assigned to the assignee of the present invention. Nevertheless, higher powers than those which may be obtained by the use of field effect transistors would be desirable.
Recent developments in device technology have provided major gains in the development of high performance heterojunction bipolar transistors. As generally known, a bipolar transistor includes a pair of junction regions formed by interposing a base region of a first carrier polarity between a pair of collector and emitter regions of a second carrier polarity. Thus, p-n-p and n-p-n structures are provided. That is, semiconductor material being doped with P-type carriers or holes and N-type carriers or electrons are provided. In silicon based technologies, known dopants for P-type and N-type materials are readily available and quite acceptable. However, silicon is not a preferred material for use of microwave frequencies because it has a relatively low saturation carrier velocity. At microwave frequencies, materials of choice are Group III-V materials, such as gallium arsenide. With gallium arsenide, a problem exists in attempting to provide a bipolar transistor since the bipolar transistor requires the use at least one P-type layer. There are no known suitable P-type dopant materials for Group III-V materials, such as gallium arsenide, since the known P-type dopant materials generally have very low saturation carrier velocities and mobilities in comparison to the known N-type dopant materials. These drawbacks, therefore, prevent devices incorporating P-type dopants from exhibiting the relatively high frequency operation performance commonly expected with the use of Group III-V materials.
However, the development of the heterojunction bipolar transistor has obviated many of these problems. In general, the heterojunction bipolar transistor includes a heterojunction comprised of a wide bandgap Group III-V material, such as aluminum gallium arsenide, which acts as an emitter layer and a lower bandgap material, such as gallium arsenide or indium gallium arsenide, which acts as a base layer. Since the bandgap difference between the two layers is relatively large by use of the higher bandgap aluminum gallium arsenide material, the base layer can be more heavily doped with P-type material for an n-p-n device, for example, thereby compensating substantially for the general lack of suitable performance characteristics of P-type dopants in gallium arsenide.
Heterojunction bipolar transistors would be widely acceptable for use in distributed amplifier applications particularly since the heterojunction bipolar transistor generally has higher power capabilities with higher gain than the MESFET. However, the heterojunction bipolar transistor has similar problems as bipolar transistors. Generally, the heterojunction bipolar transistor has a relatively low input impedance characteristic which is primarily resistive rather than capacitive. Therefore, in order to use heterojunction bipolar transistors in applications particularly requiring broadband and high output power, a different configuration or a different approach for implementation of a distributed circuit needs to be used.